Pressure sensor and manufacturing method therefor

ABSTRACT

A pressure sensor (e.g., a condenser microphone) includes a plate having a fixed electrode, a diaphragm having a moving electrode positioned opposite to the fixed electrode, and a support, wherein the diaphragm is subjected to displacement due to pressure variations applied thereto, and the support has a first interior wall forming a first cavity, in which the end portions of the plate are fixed, and a second interior wall, in which a step portion is formed in the thickness direction of the diaphragm in relation to the first interior wall and which forms a second cavity whose cross-sectional area is larger than the cross-sectional area of the first cavity in the plane direction of the diaphragm. The first and second cavities can be redesigned to communicate with each other via a passage, whereby it is possible to improve both of low-frequency characteristics and high-frequency characteristics in the pressure sensor.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/825,797, filed Jul. 9, 2007, which in turn claims priority onJapanese Patent Application No. 2006-189021, Japanese Patent ApplicationNo. 2006-196578, and Japanese Patent Application No. 2006-211889, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pressure sensors such as condensermicrophones (or silicon capacitor microphones), which are manufacturedby way of semiconductor device manufacturing processes. The presentinvention also relates to manufacturing methods of pressure sensors.

2. Description of the Related Art

Conventionally, various types of pressure sensors such as pressuresensors of silicon capacitor types and condenser microphones, which canbe manufactured by way of semiconductor device manufacturing processes,have been developed. A typical example of a pressure sensor of a siliconcapacitor type is constituted of a diaphragm that vibrates due topressure variations, a plate that is positioned opposite to thediaphragm via a dielectric such as air, and an air chamber (or acavity). Electrostatic capacitance formed between the diaphragm and theplate varies due to vibration of the diaphragm. The pressure sensorconverts variations of electrostatic capacitance into electric signals.The air chamber releases variations of internal pressure disturbingvibration of the diaphragm. Therefore, it is possible to improve outputcharacteristics of the pressure sensor by increasing the volume of theair chamber.

Japanese Patent Application Publication No. 2004-537182 teaches aminiature silicon condenser microphone in which a cavity is formedbetween a recess of a substrate and a diaphragm covering the recess,wherein the internal wall of the recess is formed perpendicular to thediaphragm; hence, the opening of the recess cannot be increased to belarger than a thin film forming the diaphragm, and it is very difficultto form the cavity having a relatively large volume. Japanese UnexaminedPatent Application Publication No. 2004-356707 teaches a condensermicrophone in which a cavity is formed by means of a diaphragm and aninternal wall of a through-hole formed in a substrate, wherein thethrough-hole is formed in a tapered shape whose diameter is increased ina direction opposite to a plate, so that the volume of the cavity can beincreased to be larger than the volume of the cavity of theaforementioned miniature silicon condenser microphone. The tapered shapeof the through-hole is formed using the lattice plane of silicon; hence,the tapered angle thereof is constant. This limits the volume of thecavity depending upon the size of a thin film forming the diaphragm;hence, it is very difficult to increase the volume of the cavity withoutincreasing the size of the pressure sensor.

In the condenser microphone taught in Japanese Patent ApplicationPublication No. 2004-537182 in which the cavity has a constant volume,high-frequency characteristics are degraded when the volume of thecavity is increased in order to improve low-frequency characteristics,while low-frequency characteristics are degraded when the volume of thecavity is decreased in order to improve high-frequency characteristics.

In the condenser microphone taught in Japanese Unexamined PatentApplication Publication No. 2004-356707, a through-hole is formed inconformity with the two-dimensional shape of the diaphragm. This isbecause the through-hole serves as an introduction path for introducingan etching solution during the wet etching process, in which theprescribed portion of a sacrifice film between the thin film forming thediaphragm and the substrate in proximity to the through-hole is removedso as to form the diaphragm above the through-hole of the substrate. Inthe wet etching process, a part of the sacrifice film formed on thesubstrate is selectively removed from the substrate by way of wetetching, thus forming prescribed parts of the condenser microphone.

Due to the shape of an opening formed in the backside of the substrateopposite to the diaphragm, bubbles may occur to entirely cover theopening of the backside of the substrate during the wet etching process,thus preventing the etching solution from entering into thethrough-hole. Since the opening of the backside of the substrate of theaforementioned condenser microphone has a circular shape in conformitywith the two-dimensional shape of the diaphragm, bubbles may easilyremain in the opening due to surface tension exerted uniformly onbubbles having semispherical shapes. This makes it necessary toartificially break remaining bubbles in the aforementioned condensermicrophone, which thus suffers from the complexity of the manufacturingprocess.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a pressure sensor,i.e., a condenser microphone that can be manufactured by way ofmanufacturing processes of semiconductor devices.

It is another object of the present invention to provide a pressuresensor having a relatively large volume of a cavity and a manufacturingmethod therefor.

It is a further object of the present invention to provide a pressuresensor, which is improved in low-frequency characteristics andhigh-frequency characteristics, and a manufacturing method therefor.

It is a further object of the present invention to provide a pressuresensor that can be manufactured with a simple manufacturing method.

In a first aspect of the present invention, a pressure sensor includes aplate having a fixed electrode, a diaphragm having a moving electrodepositioned opposite to the fixed electrode, which is subjected todisplacement due to pressure variations applied thereto, and a supporthaving a first interior wall forming a first cavity, in which endportions of the plate are fixed, and a second interior wall, in which astep portion is formed in a thickness direction of the diaphragm inrelation to the first interior wall and which forms a second cavitywhose cross-sectional area is larger than the cross-sectional area ofthe first cavity in the plane direction of the diaphragm. That is, thecross-sectional area of the second cavity is discontinuously enlarged tobe larger than the cross-sectional area of the first cavity in the planedirection of the diaphragm, and the end portions of the plate are fixedto the first interior wall forming the first cavity. This makes itpossible to increase the overall volume of the cavities of the pressuresensor without bearing limitation due to the size of the plate in itsplane direction and without enlarging the overall size of the pressuresensor.

In the above, the cross-sectional area of the second cavity in the planedirection of the diaphragm is enlarged in the direction opposite to theplate by way of the step portion, which the second interior wall formsin the thickness direction of the diaphragm; hence, it is possible toincrease the overall volume of the cavities of the pressure sensor.

In the manufacturing method of the pressure sensor, a thin film formingthe plate having the fixed electrode and a thin film forming thediaphragm are deposited on a first surface of a substrate; a first maskhaving a first opening is formed on a second surface opposite to thefirst surface of the substrate; a second mask having a second opening isformed on the second surface of the substrate, wherein the second maskcovers the first mask so that the prescribed portion of the substratejust above the thin film forming the plate is exposed in the secondopening; a recess is formed by performing anisotropic etching on thesubstrate exposed in the second opening by use of the second mask; thesecond mask is removed; then, anisotropic etching is performed using thefirst mask on the substrate exposed in the first opening such that thebottom of the recess is removed, thus forming a through-hole forming thecavity in the substrate.

When anisotropic etching is performed using the second mask on thesubstrate having the second opening, the recess is formed in thesubstrate. When anisotropic etching is performed using the first mask onthe recess of the substrate, which is exposed in the first opening, andits peripheral portion such that the bottom of the recess is removed,the through-hole having the step portion is formed in the substrate inits thickness direction. The second opening is subjected to patterningsuch that the prescribed portion of the substrate just above the thinfilm forming the plate is exposed; hence, the remaining portion of thethin film forming the plate is still deposited on the substrate aftercompletion of the formation of the through-hole. As a result, it ispossible to produce a pressure sensor having a relatively large volumeof cavities by way of the formation of the through-hole having the stepportion in the substrate.

The manufacturing method can be modified in such a way that after thedeposition of the thin film forming the plate and the thin film formingthe diaphragm on the first surface of the substrate, a mask is formed onthe second surface opposite to the first surface of the substrate,wherein the mask has a first opening for exposing the prescribed portionof the substrate just above the thin film forming the plate and a secondopening having a slit-like shape that lies along the periphery of thefirst opening; anisotropic etching is performed using the mask on thesubstrate exposed in the first and second openings, thus forming a holecorresponding to the first opening and a recess corresponding to thesecond opening in the substrate; then, a wall between the hole and therecess is removed, thus forming a through-hole forming the cavity in thesubstrate.

When anisotropic etching is performed using the mask having the firstand second openings such that the hole is formed in conformity with thefirst opening of the substrate, the recess is correspondingly formed inconformity with the second opening of the substrate due to theaspect-dependent etching effect. Then, the through-hole having the stepportion is formed in the substrate by removing the wall between the holeand the recess. The first opening is subjected to patterning so as toexpose the prescribed portion of the substrate just above the thin filmforming the plate; hence, the remaining portion of the thin film formingthe plate is still deposited on the substrate after completion of theformation of the through-hole. As a result, it is possible to produce apressure sensor having a relatively large volume of the cavity by way ofthe formation of the through-hole having the step portion in thesubstrate.

In the above, it is possible to form a plurality of second openingswhose widths are reduced in the direction departing from the firstopening, wherein the wall between the hole and its adjacent recess aswell as the wall between the recesses are simultaneously removed. Sincethe widths of the second openings are reduced in the direction departingfrom the first opening, the depths of the recesses (corresponding to thesecond openings) are reduced in the direction departing from the firstopening. This realizes the formation of the through-hole having multiplestep portions by removing the walls in the substrate; thus, it ispossible to produce a pressure sensor having a relatively large volumeof the cavity by way of the formation of the through-hole havingmultiple step portions in the substrate.

In the above, the hole and the recesses are each formed in a reverselytapered shape in the direction from the second surface to the firstsurface of the substrate, whereby thin portions of the walls are easilyisolated from the substrate so that the walls can be removed from thesubstrate with ease.

The manufacturing method can be further modified in such a way thatafter the deposition of the thin film forming the plate and the thinfilm forming the diaphragm on first surface of the substrate, a mask isformed on the second surface opposite to the first surface of thesubstrate, wherein the mask has a first opening for exposing theprescribed portion of the substrate just above the thin film forming theplate and a second opening having a slit-like shape interconnected tothe first opening; the, anisotropic etching is performed using the maskon the substrate exposed in the first and second openings, thus forminga through-hole forming the cavity in the substrate.

When anisotropic etching is performed using the mask having the firstand second openings such that the hole is formed in conformity with thefirst opening in the substrate, the recess having the slit-like shape isformed in conformity with the second opening due to the aspect-dependentetching effect in the substrate. The recess is interconnected to thehole of the substrate so that the through-hole has a step portion. Thefirst opening is subjected to patterning so as to expose the prescribedportion of the substrate just above the thin film forming the plate;therefore, the remaining portion of the thin film forming the plate isstill deposited on the substrate after completion of the formation ofthe through-hole. Thus, it is possible to produce a pressure sensorhaving a relatively large volume of the cavity by way of the formationof the through-hole having the step portion in the substrate.

In a second aspect of the present invention, a pressure sensor includesa plate having a fixed electrode, a diaphragm that has a movingelectrode positioned opposite to the fixed electrode and that issubjected to displacement due to pressure variations applied thereto, asupport having an interior wall fixed to end portions of the plate, inwhich a first cavity is formed inwardly of the interior wall of thesupport and the diaphragm, and a sub-cavity forming portion for forminga second cavity communicating with the first cavity via a passage havingan opening communicating the first cavity.

When the frequency of pressure variations increases, the diaphragm issubjected to displacement in response to high frequency, so that theinternal pressure of the first cavity varies at high frequencies. Thisincreases the velocity of an air flow of the passage. When the frequencyof pressure variations decreases, the diaphragm is subjected todisplacement at low frequency, so that the internal pressure of thefirst cavity varies at low frequency. This decreases the velocity of anair flow of the passage. Herein, the resistance of the passage increasesin response to the velocity of the air flow. For this reason, when thefrequency of pressure variations increases, substantially no air flowoccurs between the first cavity and the second cavity; hence, theoverall volume of the cavity of the pressure sensor can be substantiallyregarded as the volume of the first cavity. When the frequency ofpressure variations decreases, an air flow occurs between the firstcavity and the second cavity; hence, the overall volume of the cavity ofthe pressure sensor can be substantially regarded as the sum of thevolumes of the first and second cavities. Since the overall volume ofthe cavity of the pressure sensor is varied in response to the frequencyof pressure variations, it is possible to improve both of high-frequencycharacteristics and low-frequency characteristics of the pressuresensor.

In the above, the sub-cavity forming portion is arranged in the support,wherein the passage and the second cavity are formed inwardly of arecess of the support so as to simplify the constitution of the pressuresensor.

In addition, the sub-cavity forming portion forms a plurality of secondcavities and a plurality of passages having different resistances, viawhich the first cavity communicates with the second cavities, by whichit is possible to delicately adjust the output characteristics of thepressure sensor by individually setting the resistances of the passagesin response to required output characteristics.

Furthermore, it is possible to form a plurality of second cavitieshaving different volumes. This makes it possible to delicately adjustoutput characteristics of the pressure sensor by individually settingthe volumes of the second cavities in response to required outputcharacteristics.

In the manufacturing method of the pressure sensor, a thin film formingthe plate and a thin film forming the diaphragm are deposited on a firstsurface of a substrate forming the support; a mask is formed on a secondsurface opposite to the first surface of the substrate, wherein the maskincludes a first opening for exposing the prescribed portion of thesubstrate just above the thin film forming the plate and the thin filmforming the diaphragm, a second opening having a slit-like shape, and athird opening having a slit-like shape, which is elongated from thefirst opening to the second opening; and anisotropic etching isperformed using the mask on the substrate so as to form a hole inconformity with the first opening of the substrate, a first recess inconformity with the second opening on the second surface of thesubstrate, and a second recess, which is elongated from the hole to thefirst recess, in conformity with the third opening on the second surfaceof the substrate, thus forming a cavity forming portion in thesubstrate.

In the mask, the second opening and third opening have slit-like shapes,and the third opening is elongated from the first opening to the secondopening. When anisotropic etching is performed using the mask having thefirst, second, and third openings on the substrate such that the hole isformed in conformity with the first opening in the substrate, it ispossible to form the first cavity in correspondence with the hole of thesubstrate, and it is possible to form the sub-cavity forming portion(constituted of the first and second recesses) on the second surface ofthe substrate in correspondence with the second and third openings dueto the aspect-dependent etching effect. That is, by way of simpleprocesses using semiconductor device manufacturing processes, it ispossible to produce a pressure sensor having the first cavity and thesecond cavity, which communicates with the first cavity via the passage.

Incidentally, the manufacturing method can be modified such that theanisotropic etching is performed using the mask including a plurality ofsecond openings on the substrate so as to form a plurality of firstrecesses in the substrate; then, at least one wall between the firstrecesses positioned adjacent to each other is removed. That is, it ispossible to increase the volume of the second cavity by connecting thefirst recesses.

In a third aspect of the present invention, a pressure sensor includes asubstrate having a first surface and a second surface, which arepositioned opposite to each other, a plate having a fixed electrode,which is constituted of a thin film formed on the first surface of thesubstrate, a diaphragm that has a moving electrode positioned oppositeto the fixed electrode and that is constituted of a thin film formed onthe first surface of the substrate and is subjected to displacement dueto pressure variations applied thereto, a support constituted of a thinfilm, which is composed of a material that can be selectively removedfrom the substrate by way of wet etching and which is formed on thefirst surface of the substrate, wherein the support supports the platesuch that a gap is formed between the fixed electrode and the movingelectrode, a through-hole that is formed to run through the substrate inits thickness direction so as to expose the diaphragm and that has afirst opening, which is formed on the first surface of the substrate inconformity with the two-dimensional shape of the diaphragm, and a secondopening whose shape is substantially identical to the shape of the firstopening and which is formed on the second surface of the substrate, anda recess, which is formed on the second surface of the substrate andwhich forms a third opening communicating with the second opening in itsperiphery.

In the above, the through-hole and the recess form an inlet of anetching solution by way of wet etching, wherein the through-hole formsthe second opening on the second surface of the substrate in conformitywith the two-dimensional shape of the diaphragm, and the recess formsthe third opening projecting externally of the periphery of the secondopening on the second surface of the substrate. That is, the second andthird openings form an inlet of an etching solution on the secondsurface of the substrate. Even when bubbles occur to entirely cover theinlet on the second surface of the substrate during wet etching, surfacetensions are unevenly distributed to bubbles due to the third opening,so that bubbles may be easily burst. This simplifies the manufacturingprocess of the pressure sensor. Since the second opening of the secondsurface of the substrate is formed using the recess that is not openedin the first surface of the substrate, only the first opening is formedon the first surface of the substrate in conformity with thetwo-dimensional shape of the diaphragm; hence, it is possible to preventoutput characteristics of the pressure sensor from being degraded.

The pressure sensor can be modified to include a first through-hole andsecond through-hole, both of which are formed to run through thesubstrate in its thickness direction, in addition to the substrate,plate, diaphragm, and support. The first through-hole exposing thediaphragm has a first opening, which is formed on the first surface ofthe substrate in conformity with the two-dimensional shape of thediaphragm, and a second opening whose shape is substantially identicalto the shape of the first opening and which is formed on the secondsurface of the substrate. The second through-hole forms a third openingcommunicating with the first opening in its periphery on the firstsurface of the substrate and a fourth opening whose shape issubstantially identical to the shape of the third opening on the secondsurface of the substrate.

In the above, the first and second through-holes form an inlet of anetching solution by way of wet etching, wherein the first through-holeforms the second opening on the second surface of the substrate inconformity with the two-dimensional shape of the diaphragm, while thesecond through-hole forms the fourth opening communicating with theperiphery of the second opening on the second surface of the substrate.The second and fourth openings of the second surface of the substrateform an inlet of an etching solution. Even when bubbles occur toentirely cover the inlet during wet etching, surface tensions areunevenly distributed to bubbles due to the fourth opening; hence,bubbles may be easily burst. This simplifies the manufacturing processof the pressure sensor. In addition, the first and second through-holesform the first and third openings on the first surface of the substrate,wherein the first opening is basically shaped in conformity with thetwo-dimensional shape of the diaphragm. This makes it possible toprevent output characteristics of the pressure sensor from beingdegraded by appropriately designing the shape of the second through-holeforming the third opening.

In a manufacturing method of the pressure sensor, a sacrifice filmforming the support is deposited on a first surface of a substrate byway of wet etching using a material that can be selectively removed fromthe substrate; a thin film forming the diaphragm is deposited on thesacrifice film; a mask is formed on a second surface opposite to thefirst surface of the substrate, wherein the mask has a first openingthat is formed to expose the prescribed portion of the substrate justabove the thin film in conformity with the two-dimensional shape of thediaphragm and a second opening having a slit-like shape that iselongated externally of the periphery of the first opening; anisotropicetching is performed using the mask on the substrate so as to form athrough-hole in correspondence with the first opening of the substrateand a recess in correspondence with the second opening of the substrate;then, wet etching is performed using an etching solution, which issupplied from the through-hole of the substrate, so as to selectivelyremove the sacrifice film.

The manufacturing method can be modified such that anisotropic etchingis performed using the mask on the substrate so as to form a firstthrough-hole in correspondence with the first opening of the substrateand a second through-hole in correspondence with the second opening ofthe substrate; then, wet etching is performed using an etching solution,which is supplied from the first and second through-holes of thesubstrate, so as to selectively remove the sacrifice film.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, aspects, and embodiments of the presentinvention will be described in more detail with reference to thefollowing drawings, in which:

FIG. 1A is a plan view showing the constitution of a condensermicrophone in accordance with a first embodiment of the presentinvention;

FIG. 1B is a cross-sectional view taken along line B1-B1 in FIG. 1A;

FIG. 2A is a cross-sectional view for explaining a first step of amanufacturing method of the condenser microphone;

FIG. 2B is a plan view corresponding to FIG. 2A;

FIG. 2C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 2D is a plan view corresponding to FIG. 2C;

FIG. 2E is a cross-sectional view for explaining a third step of themanufacturing method of the condenser microphone;

FIG. 2F is a plan view corresponding to FIG. 2E;

FIG. 3A is a cross-sectional view for explaining a fourth step of amanufacturing method of the condenser microphone;

FIG. 3B is a plan view corresponding to FIG. 3A;

FIG. 3C is a cross-sectional view for explaining a fifth step of themanufacturing method of the condenser microphone;

FIG. 3D is a plan view corresponding to FIG. 3C;

FIG. 3E is a cross-sectional view for explaining a sixth step of themanufacturing method of the condenser microphone;

FIG. 3F is a plan view corresponding to FIG. 3E;

FIG. 4A is a plan view showing the constitution of a condensermicrophone according to a first variation of the first embodiment;

FIG. 4B is a cross-sectional view taken along line B4-B4 in FIG. 4A;

FIG. 5A is a cross-sectional view for explaining a first step of amanufacturing method of the condenser microphone;

FIG. 5B is a plan view corresponding to FIG. 5A;

FIG. 5C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 5D is a plan view corresponding to FIG. 5C;

FIG. 5E is a cross-sectional view for explaining a third step of themanufacturing method of the condenser microphone;

FIG. 5F is a plan view corresponding to FIG. 5E;

FIG. 5G is a cross-sectional view for explaining a fourth step of themanufacturing method of the condenser microphone;

FIG. 5H is a plan view corresponding to FIG. 5G;

FIG. 6A is a cross-sectional view for explaining a first step of amanufacturing method of a condenser microphone according to a secondvariation of the first embodiment;

FIG. 6B is a plan view corresponding to FIG. 6A;

FIG. 6C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 6D is a plan view corresponding to FIG. 6C;

FIG. 6E is a cross-sectional view for explaining a third step of themanufacturing method of the condenser microphone;

FIG. 6F is a plan view corresponding to FIG. 6E;

FIG. 7A is a cross-sectional view taken along line A7-A7 in FIG. 7B;

FIG. 7B is a bottom view showing the constitution of a condensermicrophone in accordance with a third variation of the first embodiment;

FIG. 8A is a bottom view for explaining a manufacturing method of thecondenser microphone;

FIG. 8B is a cross-sectional view corresponding to FIG. 8A;

FIG. 9 is a cross-sectional view showing the constitution of a condensermicrophone mounted on a printed board in accordance with a secondembodiment of the present invention;

FIG. 10A is a cross-sectional view taken along line A2-A2 in FIG. 10B;

FIG. 10B is a bottom view of the condenser microphone;

FIG. 11A is a cross-sectional view for explaining a first step of amanufacturing method of the condenser microphone;

FIG. 11B is a plan view corresponding to FIG. 11A;

FIG. 11C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 11D is a plan view corresponding to FIG. 11C;

FIG. 11E is a cross-sectional view for explaining a third step of themanufacturing method of the condenser microphone;

FIG. 11F is a plan view corresponding to FIG. 11E;

FIG. 12A is a cross-sectional view for explaining a fourth step of themanufacturing method of the condenser microphone;

FIG. 12B is a plan view corresponding to FIG. 12A;

FIG. 12C is a cross-sectional view for explaining a fifth step of themanufacturing method of the condenser microphone;

FIG. 12D is a plan view corresponding to FIG. 12C;

FIG. 13A is a cross-sectional view for explaining a first step of amanufacturing method of a condenser microphone according to a variationof the second embodiment;

FIG. 13B is a plan view corresponding to FIG. 13A;

FIG. 13C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 13D is a plan view corresponding to FIG. 13C;

FIG. 14A is a cross-sectional view taken along line A1-A1 in FIG. 14B,which shows the constitution of a condenser microphone in accordancewith a third embodiment of the present invention;

FIG. 14B is a bottom view of the condenser microphone;

FIG. 15A is a cross-sectional view for explaining a first step of amanufacturing method of the condenser microphone;

FIG. 15B is a plan view corresponding to FIG. 15A;

FIG. 15C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 15D is a plan view corresponding to FIG. 15C;

FIG. 15E is a cross-sectional view for explaining a third step of themanufacturing method of the condenser microphone;

FIG. 15F is a plan view corresponding to FIG. 15E;

FIG. 16A is a cross-sectional view for explaining a fourth step of themanufacturing method of the condenser microphone;

FIG. 16B is a plan view corresponding to FIG. 16A;

FIG. 16C is a cross-sectional view for explaining a fifth step of themanufacturing method of the condenser microphone;

FIG. 16D is a plan view corresponding to FIG. 16C;

FIG. 16E is a cross-sectional view for explaining a sixth step of themanufacturing method of the condenser microphone;

FIG. 16F is a plan view corresponding to FIG. 16E;

FIG. 17A is a cross-sectional view taken along line A4-A4 in FIG. 17B,which shows the constitution of a condenser microphone according to afirst variation of the third embodiment;

FIG. 17B is a bottom view corresponding to FIG. 17A;

FIG. 18A is a cross-sectional view for explaining a first step of amanufacturing method of the condenser microphone;

FIG. 18B is a plan view corresponding to FIG. 18A;

FIG. 18C is a cross-sectional view for explaining a second step of themanufacturing method of the condenser microphone;

FIG. 18D is a plan view corresponding to FIG. 18C;

FIG. 19A is a bottom view showing the constitution of a condensermicrophone according to a second variation of the third embodiment; and

FIG. 19B is a bottom view showing the constitution of a condensermicrophone according to a third variation of the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in further detail by way ofexamples with reference to the accompanying drawings.

1. First Embodiment

FIGS. 1A and 1B show the constitution of a condenser microphone 1 suchas a silicon capacitor microphone that is manufactured by way of thesemiconductor device manufacturing process and a pressure sensor thatconverts sound waves transmitted to a plate 21 into electric signals.

A sensing portion of the condenser microphone 1 has a laminatedstructure composed of a substrate 10, a first film, a second film, athird film, and a fourth film. The substrate 10 is a mono-crystalsilicon substrate, in which a through-hole 12 having a step portion isformed in a thickness direction.

The first film is an insulating thin film composed of silicon dioxide.The first film supports the second film above the substrate 10 so as toform a gap between a diaphragm 20 and the substrate 10. A circularopening 13 is formed in the first film.

The second film is a conductive thin film composed of polysilicon dopedwith phosphorus impurities (P). The prescribed portion of the secondfilm that is not fixed to the third film forms the diaphragm 20. Thediaphragm 20 is not fixed to either the first film or the third film,wherein it serves as a moving electrode that vibrates due to soundwaves. The diaphragm 20 has a circular shape covering the opening 13 ofthe first film.

Similar to the first film, the third film is an insulating thin filmcomposed of silicon dioxide. The third film insulates the fourth filmfrom the second film having a conductivity so as to support the fourthfilm above the second film. A circular opening 15 is formed in the thirdfilm.

Similar to the second film, the fourth film is a conductive thin filmcomposed of polysilicon doped with phosphorus impurities (P). Theprescribed portion of the fourth film that is not fixed to the thirdfilm forms a plate 21. A plurality of holes 22 are formed in the plate21.

A support 23 is constituted of the substrate 10, the first film, thethird film, the second film, and the prescribed portion of the fourthfilm that is not fixed to the third film. A back cavity 40 constitutedof a first cavity 41 and second cavity 42 is formed in the support 23.The back cavity 40 releases pressure that is applied to the diaphragm 20in a direction opposite to the propagation direction of sound waves. Thefirst cavity 41 is formed inwardly of an interior wall 12 a of thesubstrate 10, which is positioned in proximity to the plate 21, andinwardly of an interior wall 13 a of the opening 13 of the second film.The second cavity 42 is formed inwardly of an interior wall 12 b of thesubstrate 10, which is positioned opposite to the plate 21. Thecross-sectional area of the second cavity 42 lying in the planedirection of the diaphragm 20 is larger than that of the first cavity41. In the claim language, both of the interior wall 12 a of thesubstrate 10 and the interior wall 13 a of the second film are definedas a first interior wall, and the interior wall 12 b of the substrate 10is defined as a second interior wall.

A detecting portion of the condenser microphone 1 will be described byway of the circuitry shown in FIG. 1B, in which the diaphragm 20 isconnected to a bias voltage source. Specifically, leads 104 and 106 areconnected to a terminal 102 of the bias voltage source, wherein the lead104 is connected to the substrate 10, and the lead 106 is connected tothe second film, so that both of the diaphragm 20 and the substrate 10are placed at the substantially same potential. In addition, the plate21 is connected to an input terminal of an operational amplifier 100.That is, a lead 108 connected to the input terminal of the operationalamplifier 100 is connected to the fourth film. The operational amplifier100 has high input impedance.

Next, the operation of the condenser microphone 1 will be described.When sound waves are transmitted to the diaphragm 20 via the holes 22 ofthe plate 21, the diaphragm 20 vibrates due to sound waves. Thevibration of the diaphragm 20 causes variations of the distance betweenthe diaphragm 20 and the plate 21, thus varying electrostaticcapacitance formed between the diaphragm 20 and the plate 21.

Since the plate 21 is connected to the operational amplifier 100 havinghigh input impedance, a very small amount of electric charge existing inthe plate 21 moves toward the operational amplifier 100 irrespective ofvariations of electrostatic capacitance between the diaphragm 20 and theplate 21. Therefore, it can be presumed that electric charges existingin the plate 21 and the diaphragm 20 may not change. This makes itpossible to translate variations of electrostatic capacitance betweenthe diaphragm 20 and the plate 21 into potential variations of the plate21. Thus, the condenser microphone 1 is capable of producing electricsignals based on very small variations of electrostatic capacitancebetween the diaphragm 20 and the plate 21. That is, the condensermicrophone 1 converts variations of sound pressure applied to thediaphragm 20 into variations of electrostatic capacitance, which arethen converted into voltage variations, based on which electric signalsare produced in response to variations of sound pressure.

Next, a manufacturing method of the condenser microphone 1 will bedescribed with reference to FIGS. 2A to 2F and FIGS. 3A to 3F.

In a first step of the manufacturing method (i.e., (A1) and (B1) shownin FIGS. 2A and 2B), a first film 51 is deposited on a wafer 50 formingthe substrate 10 (see FIG. 1B). That is, silicon dioxide is deposited onthe wafer 50 composed of monocrystal silicon by way of plasma chemicalvapor deposition (or plasma CVD), thus forming the first film 51.

Next, a second film 52 is deposited on the first film 51. That is,phosphorus-doped polysilicon is deposited on the first film 51 by way ofdecompression CVD, thus forming the second film 52. Next, a photoresistfilm is applied to the entire surface of the second film 52; then, aresist pattern is formed by way of photolithography for performingexposure and development using a prescribed resist mask. Then, thesecond film 52 is selectively removed by way of anisotropic etching suchas reactive ion etching (RIE), thus forming the second film 52 having acircular shape.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 2C and 2D), a third film 53 is deposited on the second film 52.That is, silicon dioxide is deposited on the second film 52 by way ofplasma CVD, thus forming the third film 53.

In a third step of the manufacturing method (i.e., (A3) and (B3) shownin FIGS. 2E and 2F), a fourth film 54 is deposited on the third film 53.That is, phosphorus-doped polysilicon is deposited on the third film 53by way of decompression CVD, thus forming the fourth film 54. Next, aphotoresist film is applied to the entire surface of the fourth film 54;then, a resist pattern is formed by way of photolithography forperforming exposure and development using a prescribed resist mask.Then, the fourth film 54 is selectively removed by way of anisotropicetching such as RIE, thus forming the fourth film 54 having a circularshape and the plurality of holes 22.

In a fourth step of the manufacturing method (i.e., (A4) and (B4) shownin FIGS. 3A and 3B), a first mask 55 having a first opening 55 a isformed on a second surface 50 b opposite to a first surface 50 a of thewafer 50 on which the first film 51, the second film 52, the third film53, and the fourth film 54 are laminated. That is, the first mask 55composed of a metal is adhered to the wafer 50 by use of the adhesive.It is preferable that the adhesive be an organic adhesive, and it ispreferable that the first mask 55 be composed of nickel or chromium.Incidentally, the first mask 55 can be composed of any types ofmaterials as long as it can be selectively removed together with asecond mask 56. Alternatively, the first mask 55 can be formed byperforming metal plating on the wafer 50.

Next, a second mask 56 having a second opening 56 a is formed on thesecond surface 50 b of the wafer 50 and the first mask 55. That is, aphotoresist film is applied to the entire surface corresponding to thesecond surface 50 b of the wafer 50 and the first mask 55; then, thesecond mask 56 is formed by way of photolithography for performingexposure and development using a prescribed resist mask. Herein, thesecond opening 56 a exposes the prescribed portion of the wafer 50 justabove the second film 52 and the fourth film 54 in the first opening 55a.

In a fifth step of the manufacturing method (i.e., (A5) and (B5) shownin FIGS. 3C and 3D), the prescribed portion of the wafer 50 exposedinwardly of the second opening 56 a is subjected to anisotropic etchingusing the second mask 56, thus forming a recess 60 in the wafer 50. Thatis, the wafer 50 is selectively removed by way of Deep-RIE, thus formingthe recess 60 in the wafer 50.

In a sixth step of the manufacturing method (i.e., (A6) and (B6) shownin FIGS. 3E and 3F), the second mask 56 is removed. That is, the secondmask 56 is removed by use of a resist peeling solution such as NMP(i.e., N-methyl-2-pyrolidone).

Next, the prescribed portion of the wafer 50 exposed inwardly of thefirst opening 55 a is subjected to anisotropic etching using the firstmask 55, thus forming the through-hole 12 having a step portion in thewafer 50. That is, the wafer 50 is selectively removed such that thebottom of the recess 60 disappears by way of Deep-RIE, thus forming thethrough-hole 12 having the step portion in the wafer 50. Herein, theprescribed portion of the first film 51 just above the second film 52and the fourth film 54 is exposed inwardly of the through-hole 12. Theremaining portions of the second film 52 and the fourth film 54 arestill deposited in the surrounding area of an opening of the secondsurface 50 b of the wafer 50 after the completion of the formation ofthe through-hole 12.

Next, the first film 51 and the third film 53, both of which are siliconoxide films, are selectively removed by way of isotropic etching (e.g.,wet etching) using an etching solution such as buffered hydrofluoricacid (or Buffered HF) or by way of the combination of isotropic etchingand anisotropic etching. Herein, the etching solution is supplied viathe holes 22 of the fourth film 54 and the through-hole 12 of the wafer50 so as to dissolve the first film 51 and the third film 53. Byappropriately designing the shapes and arrangements of the holes 22 andthe through-hole 12, it is possible to form the openings 13 and 15 inthe first film 51 and the third film 53. Thus, it is possible to formthe sensing portion constituted of the diaphragm 20, the plate 21, andthe support 23 (see FIGS. 1A and 1B).

Thereafter, other steps such as dicing and packaging are performed so asto completely produce the condenser microphone 1.

The first embodiment can be modified in a variety of ways; hence,variations will be described below.

(a) First Variation

FIGS. 4A and 4B show the constitution of a condenser microphone 2 inaccordance with a first variation of the first embodiment. The sensingportion of the condenser microphone 2 differs from the sensing portionof the condenser microphone 1 in terms of the shape of the substrate 10.Specifically, a through-hole 212 having multiple step portions is formedin the substrate 10 of the condenser microphone 2. Herein, a support 223is constituted of the substrate 10, the first film, the third film, andthe prescribed portions of the second and fourth films that are notfixed to the third film.

A back cavity 240 constituted of a first cavity 241 and a second cavity242 is formed in the support 223. The first cavity 241 is formedinwardly of an interior wall 212 a of the substrate 10 positioned inproximity to the plate 21 and inwardly of the interior wall 13 a of theopening 13 of the second film. The second cavity 242 is formed inwardlyof an interior wall 212 b of the substrate 10 positioned opposite to theplate 21. The interior wall 212 b of the substrate 10 forms a stepportion lying in the thickness direction of the diaphragm 20. Thus, thecross-sectional area of the second cavity 242 lying in the thicknessdirection of the diaphragm 20 is enlarged discontinuously in a directionextending oppositely from the plate 21. In the claim language, theinterior wall 212 a of the substrate 10 and the interior wall 13 a ofthe second film are both defined as a first interior wall, and theinterior wall 212 b of the substrate 10 is defined as a second interiorwall.

The detecting portion of the condenser microphone 2 is substantiallyidentical to the detecting portion of the condenser microphone 1. Hence,the operation of the condenser microphone 2 is substantially identicalto the operation of the condenser microphone 1. For the sake ofconvenience, the detailed description regarding the operation of thecondenser microphone 2 will be omitted.

Next, a manufacturing method of the condenser microphone 2 will bedescribed with reference to FIGS. 5A to 5H.

Similar to the manufacturing method of the condenser microphone 1, in afirst step (i.e., (A1) and (B1) shown in FIGS. 5A and 5B) of themanufacturing method of the condenser microphone 2, the first film 51,the second film 52, the third film 53, and the fourth film 54 aredeposited on the first surface 50 a of the wafer 50.

Next, a mask 255 having openings 255 a, 255 b, 255 c, and 255 d isformed on the second surface 50 b of the wafer 50. That is, aphotoresist film is applied entirely to the second surface 50 b of thewafer 50; then, the mask 255 is formed by way of photolithography forperforming exposure and development using a prescribed resist mask. Theopening 255 a has a circular shape that exposes a prescribed portion ofthe wafer 50 just above the second film 52 and the fourth film 54. Theopenings 255 b, 255 c, and 255 d each having a ring shape aresequentially expanded in the circumferential periphery of the opening255 a. Each of the ring-shaped openings 255 b, 255 c, and 255 d forms aslit whose width in a radial direction is smaller than the diameter ofthe opening 255 a. In view of the radial direction, the width of theopening 255 b is smaller than the width of the opening 255 b; and thewidth of the opening 255 d is smaller than the width of the opening 255c.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 5C and 5D), the wafer 50 is subjected to anisotropic etchingusing the mask 255, thus forming a hole 260 and recesses 261, 262, and263 in the wafer 50. Specifically, the prescribed portion of the wafer50, which is exposed from the mask 255, is selectively removed by way ofanisotropic etching such as Deep-RIE. The anisotropic etching iscontinuously performed until the hole 260 is completely formed topositionally match the opening 255 a in the wafer 50. Each of the widthsof the openings 255 b, 255 c, and 255 d in a radial direction is smallerthan the diameter of the opening 255 a. Due to the aspect-dependentetching effect, the recesses 261, 262, and 263 are respectively formedto positionally match the openings 255 b, 255 c, and 255 d in the wafer50. Since the width of the opening 255 c is smaller than the width ofthe opening 255 b in the radial direction, the depth of the recess 262is smaller than the depth of the recess 261 in the thickness directionof the wafer 50. Since the width of the opening 255 d is smaller thanthe width of the opening 255 c in the radial direction, the depth of therecess 263 is smaller than the depth of the recess 262 in the thicknessdirection of the wafer 50.

As shown in FIGS. 5E and 5F, a wall 271 is formed between the hole 260and the recess 261; a wall 272 is formed between the recesses 261 and262; and a wall 273 is formed between the recesses 262 and 263. Thewalls 271, 272, and 273 are removed as shown in FIGS. 5G and 5H.Specifically, the second surface 50 b of the wafer 50 composed ofmonocrystal silicon is subjected to thermal oxidation, thus transformingthe walls 271, 272, and 273 into silicon oxide; then, wet etching isperformed using an etching solution such as buffered hydrofluoric acidso as to selectively remove the walls 271, 272, and 273 together withthe transformed portion of the second surface 50 b of the wafer 50. As aresult, it is possible to form the through-hole 212 having multiple stepportions in the wafer 50, wherein the prescribed portion of the firstfilm 51 just above the second film 52 and the fourth film 54 is exposedin the through-hole 212. The remaining portions of the second film 52and the fourth film 54 are still deposited in the surrounding area ofthe opening in the second surface 50 b of the wafer 50 after completionof the formation of the through-hole 212.

Steps following the aforementioned steps of the manufacturing method ofthe condenser microphone 2 are substantially identical to those of themanufacturing method of the condenser microphone 1.

(b) Second Variation

In the manufacturing method of the condenser microphone 2 according tothe first variation of the first embodiment, the walls 271, 272, and 273of the wafer 50 are selectively removed by way of transformation. Ofcourse, the process for selectively removing walls is not necessarilylimited to the aforementioned process.

Next, a manufacturing method according to a second variation of thefirst embodiment will be described with respect to the process forselectively removing walls.

Similar to the manufacturing method of the condenser microphone 1, in afirst step (i.e., (A1) and (B1) shown in FIGS. 6A and 6B) of themanufacturing method of the condenser microphone according to the secondvariation of the first embodiment, the first film 51, the second film52, the third film 53, and the fourth film 54 are deposited on the firstsurface 50 a of the wafer 50.

Next, a mask 355 having openings 355 a and 355 b is formed on the secondsurface 50 b of the wafer 50. That is, a photoresist mask is appliedentirely to the second surface 50 b of the wafer 50; then, the mask 355is formed by way of photolithography for performing exposure anddevelopment using a prescribed resist mask. The opening 355 a has acircular shape that exposes the prescribed portion of the wafer 50 justabove the second film 52 and the fourth film 54. The opening 355 b has aring shape that is formed in the circumferential periphery of theopening 355 a, wherein the opening 355 b forms a slit whose width in aradial direction is smaller than the diameter of the opening 355 a.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 6C and 6D), the wafer 50 is subjected to anisotropic etchingusing the mask 355, thus forming a hole 360 and a recess 361 in thewafer 50. Each of the hole 360 and the recess 361 is formed in areversely tapered shape extended in a vertical direction from the secondsurface 50 b to the first surface 50 a of the wafer 50. Specifically,the prescribed portion of the wafer 50 that is exposed from the mask 355is selectively removed by way of anisotropic etching such as Deep-RIE.The anisotropic etching is performed in such a way that the hole 360 iscompletely formed to positionally match the opening 355 a of the wafer50. By adjusting etching conditions, it is possible to form the hole 360and the recess 361 each having a reversely tapered shape. For example,the wafer 50 is etched in a low deposition condition for side wallprotection films; alternatively, the wafer 50 is etched while adjustingthe formation time and etching time adapted to side wall protectionfilms. As a result, a wall 371 formed between the hole 360 and therecess 361 is gradually reduced in thickness in the vertical directionfrom the second surface 50 b to the first surface 50 a in the wafer 50.

In a third step of the manufacturing method (i.e., (A3) and (B3) shownin FIGS. 6E and 6F), the wall 371 of the wafer 50 is removed. That is,the wafer 50 is subjected to wet etching using an etching solution suchas potassium hydroxide (KOH) and tetra-methyl ammonium hydroxide (TMAH).Herein, the thin portion of the wall 371 is dissolved first comparedwith the other portion, so that the wall 371 is separated from the wafer50; then, the wall 371 isolated from the wafer 50 is completelydissolved in the etching solution. As a result, it is possible to formthe through-hole 12 having a step portion in the wafer 50. The processfor isolating the wall 371 from the wafer 50 is not necessarily limitedto the aforementioned process. For example, the wall 371 can be isolatedfrom the wafer 50 by applying ultrasonic waves or mechanical vibrationto the wall 371. Steps following the aforementioned steps aresubstantially identical to those of the manufacturing method of thecondenser microphone 1.

(c) Third Variation

FIGS. 7A and 7B show the constitution of a condenser microphone 4 inaccordance with a third variation of the first embodiment. The sensingportion of the condenser microphone 4 differs from the sensing portionof the condenser microphone 1 in terms of the shape of the substrate 10.A through-hole 412 having a step portion is formed and runs through thesubstrate 10 of the condenser microphone 4 in the thickness direction,wherein it is constituted of a hole 400 having a cylindrical shape and aplurality of recesses 401, which are formed in a radial manner in thecircumferential periphery of the hole 400 so as to directly communicatewith the hole 400. A support 423 is constituted of the substrate 10, thefirst film, the third film, and the prescribed portions of the secondand fourth films that are not fixed to the third film.

A back cavity 440 constituted of a first cavity 441 and a second cavity442 is formed in the support 423. The first cavity 441 is formedinwardly of an interior wall 412 a of the hole 400 positioned inproximity to the plate 21 and inwardly of the interior wall 13 a of theopening 13 of the second film. The second cavity 442 is formed inwardlyof an interior wall 412 b, which is constituted of an interior wall ofthe hole 400 positioned in proximity to the plate 21 and interior wallsof the recesses 401. In the claim language, the interior wall 412 a ofthe substrate 10 and the interior wall 13 a of the second film are bothdefined as a first interior wall, and the interior wall 412 b of thesubstrate 10 is defined as a second interior wall.

The detecting portion of the condenser microphone 4 is substantiallyidentical to the detecting portion of the condenser microphone 1. Theoperation of the condenser microphone 4 is substantially identical tothe operation of the condenser microphone 1. Hence, the detaileddescription regarding the operation of the condenser microphone 4 willbe omitted.

Next, a manufacturing method of the condenser microphone 4 will bedescribed with reference to FIGS. 8A and 8B.

Similar to the manufacturing method of the condenser microphone 1, thefirst film 51, the second film 52, the third film 53, and the fourthfilm 54 are formed on the first surface 50 a of the wafer 50.

Next, a mask 455 having an opening 455 a and a plurality of openings 455b is formed on the second surface 50 b of the wafer 50. That is, aphotoresist film is applied entirely to the second surface 50 b of thewafer 50; then, the mask 455 is formed by way of photolithography forperforming exposure and development using a prescribed resist mask. Theopening 455 a has a circular shape that exposes the prescribed portionof the wafer 50 just above the second film 52 and the fourth film 54.The openings 455 b form slits that are elongated from the opening 455 ain a radial manner. Each of widths of the openings 455 b lying in acircumferential direction of the opening 455 a is smaller than thediameter of the opening 455 a.

Next, the wafer 50 is subjected to wet etching using the mask 455, thusforming the through-hole 412 in the wafer 50. Specifically, theprescribed portion of the wafer 50 that is exposed from the mask 455 isselectively removed by way of anisotropic etching such as Deep-RIE. Theanisotropic etching is performed in such a way that the hole 400 iscompletely formed to positionally match the opening 455 a of the wafer50 (see FIGS. 7A and 7B). Each of widths of the openings 455 b lying ina radial direction of the opening 455 a is smaller than the diameter ofthe opening 455 a. Due to the aspect-dependent etching effect, therecesses 401 are formed to positionally match the openings 455 b of thewafer 50. As a result, it is possible to form the through-hole 412having a step portion, which is constituted of the hole 400 and theplural recesses 401, in the wafer 50, wherein the prescribed portion ofthe first film 51 just above the second film 52 and the fourth film 54is exposed in the through-hole 412. The remaining portions of the secondfilm 52 and the fourth film 54 are still deposited in the surroundingarea of the opening in the second surface 50 b of the wafer 50 aftercompletion of the formation of the through-hole 412.

Steps following the aforementioned steps are substantially identical tothose of the manufacturing method of the condenser microphone 1.

The first embodiment and its variations are designed such that, in viewof the plane direction of the diaphragm 20, the cross-sectional area ofthe second cavity is rapidly increased in comparison with thecross-sectional area of the first cavity; and the end portions of thediaphragm 20 and the plate 21 are fixed to the interior wall of thefirst cavity. Therefore, it is possible to increase the volume of theback cavity without bearing limitation due to the sizes of the diaphragm20 and the plate 21 and without increasing the overall size of thecondenser microphone.

In the first variation (see FIGS. 4A and 4B), the cross-sectional areaof the second cavity 242 in view of the plane direction of the diaphragm20 is enlarged in a step-like manner in the direction opposite to theplate 21. Since the cross-sectional area of the second cavity 242 inview of the plane direction of the diaphragm 20 is discontinuouslyenlarged in the direction opposite to the plate 21, it is possible toincrease the volume of the back cavity 240.

(d) Other Variations

The first embodiment and its variations are each directed to thecondenser microphone, which is an example of a pressure sensor. Ofcourse, the first embodiment can be applied to other types of pressuresensors that detect various kinds of pressure other than sound pressure.

The first embodiment and its variations are each directed to thecondenser microphone in which both of the diaphragm 20 and the plate 21have circular shapes whose circumferential peripheries are entirelyfixed to the support. The sensing portion of the condenser microphoneconstituted of the diaphragm and plate is not necessarily limited to theaforementioned structure. For example, the end portions of the diaphragmand plate can be partially fixed to the support. Specifically, both endsof the diaphragm can be fixed to the support; alternatively, thediaphragm can be fixed to the support in a cantilever manner. The shapesof the diaphragm and plate are not necessarily limited to circularshapes. Specifically, the diaphragm and plate can be formed in polygonalshapes. The plate can be positioned close to the back cavity rather thanthe diaphragm. The diaphragm is not necessarily directly fixed to thesupport. Specifically, the diaphragm can be attached to the plate in ahung-down manner; alternatively, the diaphragm can be supported by theplate.

The first embodiment and its variations are each designed such that athrough-hole having a step portion realizing a rectangular step portionis formed in the substrate 10, although it is not necessary to form therectangular step portion along the interior wall of the support.

In the first embodiment, first variation, and second variation, thesecond cavity is formed and is enlarged in the periphery of the firstcavity, whereby it is possible to enlarge the second cavity partiallyexternally of the first cavity.

The manufacturing method of the condenser microphone 1 can be applied tothe manufacturing of the condenser microphone 2. In this case, it isnecessary to form a multilayered mask in which the number of layersdepends upon the number of the step portions formed in the through-hole12. A multilayered mask constituted of the first mask 55 and the secondmask 56 (see FIGS. 3A, 3C, and 3E) is used in the manufacturing methodof the condenser microphone 1, although it is possible to use asingle-layered resist mask whose thickness depends upon the overallshape of the through-hole 12 having a step portion.

The manufacturing method of the condenser microphone 2 can be applied tothe manufacturing of the condenser microphone 1. In this case, it isnecessary to form the openings 255 b, 255 c, and 255 d, all of whichhave the same width in the radial direction, in the mask 255.

The manufacturing method of the condenser microphone 2 uses the mask 255having the ring-shaped openings 255 b, 255 c, and 255 d, although it ispossible to form the openings 255 b, 255 c, and 255 d each in aband-like shape.

The manufacturing method of the condenser microphone 2 can be modifiedsuch that other slit-like openings, which cross the openings 255 b, 255c, and 255 d, can be additionally formed in the mask 255. In this case,the walls of the recesses corresponding to the openings 255 a, 255 b,and 255 c are split by means of the recesses corresponding to the otheropenings in the substrate 10. This makes it easy to remove the walls byway of wet etching.

The second variation describes the manufacturing method of the condensermicrophone 1. Of course, the second variation can be applied to themanufacturing of the condenser microphone 2.

In the third variation, the plurality of recesses 401 are formed tocommunicate with the hole 400, although it is possible to form a singlerecess 401 in the periphery of the hole 400.

In the third variation, the plurality of recesses 401 are evenlydistributed in a radial manner externally of the hole 400;alternatively, it is possible to unevenly distribute the recesses 401 inthe periphery of the hole 400.

2. Second Embodiment

FIG. 9 and FIGS. 10A and 10B show the constitution of a condensermicrophone 1001 in accordance with a second embodiment of the presentinvention. The condenser microphone 1001 is a silicon capacitormicrophone that is manufactured by way of semiconductor devicemanufacturing processes, wherein it converts sound waves transmittedthereto via a plate 1022 into electric signals.

A sensing portion of the condenser microphone 1001 has a laminatedstructure constituted of a substrate 1010, a first film, a second film,a third film, and a fourth film.

The substrate 1010 is a monocrystal silicon substrate, in which a hole1011, a recess 1012, and a plurality of recesses 1013 are formed in thethickness direction. The recess 1012 has a ring shape surrounding thehole 1011. Each of the recesses 1013 has a linear shape elongated fromthe hole 1011 to the recess 1012 in a radial direction of the hole 1011.

The first film is an insulating thin film composed of silicon dioxide.The first film supports the second film above the substrate 1010 in sucha way that a gap is formed between a diaphragm 1020 and the substrate1010. An opening 1014 having a circular shape is formed in the firstfilm.

The second film is a conductive thin film composed of polysilicon dopedwith phosphorus (P) impurities. The prescribed portion of the secondfilm that is not fixed to the third film forms the diaphragm 1020. Thediaphragm 1020 is not fixed to either the first film or the third film;hence, it serves as a moving electrode vibrating due to sound waves. Thediaphragm 1020 has a circular shape covering the opening 1014 of thefirst film.

Similar to the first film, the third film is an insulating thin filmcomposed of silicon dioxide. The third film insulates the second film(having conductivity) from the fourth film so as to support the fourthfilm above the second film. The third film has an opening 1015 having acircular shape.

Similar to the second film, the fourth film is a conductive thin filmcomposed of polysilicon doped with phosphorus impurities. The prescribedportion of the fourth film that is not fixed to the third film forms theplate 1022, which has a plurality of holes 1023.

A support 1024 is constituted of the substrate 1010, the first film, thethird film, and the prescribed portions of the second and fourth filmsthat are not fixed to the third film. As shown in FIG. 9, the support1024 forms a back cavity 1040 constituted of a first cavity (or a maincavity) 1041 and a second cavity (or a sub cavity) 1042, whichcommunicates with the first cavity 1041 via a passage 1043. The backcavity 1040 releases pressure that is applied to the diaphragm 1020 in adirection opposite to a propagation direction of sound waves. The firstcavity 1041 is formed inwardly of the diaphragm 1020, the opening 1014of the second film, the hole 1011 of the substrate 1010, and a printedboard 1060 on which the condenser microphone 1001 is mounted. The secondcavity 1042 is formed inwardly of the recess 1012 of the substrate 1010and the printed board 1060. In the claim language, the recesses 1012 and1013 of the substrate 1010 are both defined as a sub-cavity formingportion, and the hole 1011 and the recesses 1012 and 1013 of thesubstrate 1010 are all defined as a cavity forming portion.

Next, a detecting portion of the condenser microphone 1001 will bedescribed with reference to the circuitry shown in FIG. 10A. Thediaphragm 1020 is connected to a bias voltage source. Specifically,leads 1104 and 1106 connected to a terminal 1102 of the bias voltagesource are connected to the second film and the substrate 1010respectively, whereby both of the diaphragm 1020 and the substrate 1010are placed at substantially the same potential. The plate 1022 isconnected to an input terminal of an operation amplifier 1100. That is,a lead 1108 connected to the input terminal of the operational amplifier1100 having relatively high input impedance is connected to the fourthfilm.

Next, the operation of the condenser microphone 1001 will be describedin detail. When sound waves are transmitted through the holes 1023 ofthe plate 1022 so as to reach the diaphragm 1020, the diaphragm 1020vibrates due to sound waves. Due to the vibration of the diaphragm 1020,the distance between the diaphragm 1020 and the plate 1022 varies sothat electrostatic capacitance therebetween varies correspondingly.

Since the plate 1022 is connected to the operational amplifier 1100having relatively high input impedance, a very small amount of electriccharge existing in the plate 1022 moves toward the operational amplifier1100 even when electrostatic capacitance between the diaphragm 1020 andthe plate 1022 varies. That is, it is presumed that substantially novariations occur in electric charges existing in the plate 1022 and thediaphragm 1020. This makes it possible to translate variations ofelectrostatic capacitance between the diaphragm 1020 and the plate 1022into potential variations of the plate 1022. As a result, the condensermicrophone 1001 is capable of producing electric signals based on verysmall variations of electrostatic capacitance between the diaphragm 1020and the plate 1022. In the condenser microphone 1001, variations ofsound pressure applied to the diaphragm 1020 are converted intovariations of electrostatic capacitance, which are then converted intopotential variations, based on which electric signals are produced inresponse to variations of sound pressure.

The internal pressure (or back pressure) of the back cavity 1040 variesdue to the vibration of the diaphragm 1020. That is, the volume of theback cavity 1040 greatly affects the vibration of the diaphragm and thusaffects output characteristics of the condenser microphone 1001.Specifically, it is possible to improve low-frequency characteristics ofthe condenser microphone 1001 by increasing the volume of the backcavity 1040, while it is possible to improve high-frequencycharacteristics of the condenser microphone 1001 by decreasing thevolume of the back cavity 1040.

The resistance of the passage 1043 allowing the first cavity 1041 tocommunicate with the second cavity 1042 in the back cavity 1040increases in response to the flow velocity of air flowing through thepassage 1043. As the frequency of variations of back pressure increases,in other words, as the frequency of the displacement of the diaphragm1020 increases due to sound waves having high frequencies, substantiallyno air flows between the first cavity 1041 and the second cavity 1042.This indicates that the volume of the back cavity 1040 can besubstantially regarded as the volume of the first cavity 1041. Incontrast, as the frequency of variations of back pressure decreases, inother words, as the frequency of the displacement of the diaphragm 1020decreases due to sound waves having low frequencies, air adequatelyflows between the first cavity 1041 and the second cavity 1042. Thisindicates that the volume of the back cavity 1040 can be substantiallyregarded as the sum of the volumes of the first cavity 1041 and thesecond cavity 1042.

Since the volume of the back cavity 1040 substantially varies inresponse to the frequency of sound waves, it is possible to improve bothof low-frequency characteristics and high-frequency characteristics inthe condenser microphone 1001. That is, output characteristics of thecondenser microphone 1001 can be adjusted by appropriately setting thevolume of the first cavity 1041, the volume of the second cavity 1042,and the resistance of the passage 1043. The resistance of the passage1043 can be set by appropriately setting the length, width, and depth ofthe recess 1013 formed in the substrate 1010. The depth of the recess1013 is not necessarily smaller than the depth of the recess 1012 in thethickness direction of the substrate 1010. As shown in FIGS. 10A and10B, the length of the recess 1013 is measured in a radial direction ofthe hole 1011; the width of the recess 1013 is measured in acircumferential direction of the hole 1011; and the depth of the recess1013 is measured in the thickness direction of the substrate 1010.

Next, a manufacturing method of the condenser microphone 1001 will bedescribed with reference to FIGS. 11A to 11F and FIGS. 12A to 12D.

In a first step of the manufacturing method (i.e., (A1) and (B1) shownin FIGS. 11A and 11B), a first film 1051 is deposited on a wafer 1050,which serves as the substrate 1010 (see FIGS. 10A and 10B).Specifically, silicon dioxide is deposited on the monocrystal siliconwafer 1050 by way of plasma CVD, thus forming the first film 1051.

Next, a second film 1052 is deposited on the first film 1051. That is,phosphorus-doped polysilicon is deposited on the first film 1051 by wayof decompression CVD, thus forming the second film 1052. Next, aphotoresist film is applied to the entire surface of the second film1052; then, a resist pattern is formed by way of photolithography forperforming exposure and development using a prescribed resist mask.Then, the second film 1052 is selectively removed by way of anisotropicetching such as RIE (i.e., Reactive Ion Etching), thus shaping thesecond film 1052 in a circular shape.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 11C and 11D), a third film 1053 is deposited on the second film1052. Specifically, silicon dioxide is deposited on the second film 1052by way of plasma CVD, thus forming the third film 1053.

In a third step of the manufacturing method (i.e., (A3) and (B3) shownin FIGS. 11E and 11F), a fourth film 1054 is deposited on the third film1053. Specifically, phosphorus-doped polysilicon is deposited on thethird film 1053 by way of decompression CVD, thus forming the fourthfilm 1054. Next, a photoresist film is applied to the entire surface ofthe fourth film 1054; then, a resist pattern is formed by way ofphotolithography for performing exposure and development using aprescribed resist mask. Then, the fourth film 1054 is selectivelyremoved by way of anisotropic etching such as RIE, thus shaping thefourth film 1054 having a circular shape and a plurality of holes 1023.

In a fourth step of the manufacturing method (i.e., (A4) and (B4) shownin FIGS. 12A and 12B), a mask 1055 having a first opening 1055 a, asecond opening 1055 b, and a plurality of third openings 1055 c isformed on a second surface 1050 b opposite to a first surface 1050 a ofthe wafer 1050 on which the first film 1051, the second film 1052, thethird film 1053, and the fourth film 1054 are laminated. That is, aphotoresist mask is applied to the entire surface of the second surface1050 b of the wafer 1050; then, the mask 1055 is formed by way ofphotolithography for performing exposure and development using aprescribed resist mask. The first opening 1055 a has a circular shapeexposing the prescribed portion of the wafer 1050 just above the secondfilm 1052 and the fourth film 1054. The second opening 1055 b has aring-shaped slit surrounding the periphery of the first opening 1055 a.Each of the third openings 1055 c is a slit whose width is smaller thanthe width of the slit-shaped second opening 1055 b. The third openings1055 c are elongated in a radial manner in a direction from the firstopening 1055 a to the second opening 1055 b. For example, the width ofthe second opening 1055 b ranges from 1 μm to 100 μm (preferably, from 1μm to 70 μm); and the width of the third opening 1055 c ranges from 1 μmto 50 μm (preferably, from 1 μm to 40 μm). In the condenser microphone1001 shown in FIGS. 10A and 10B, the width of the second opening 1055 bis measured in the radial direction of the first opening 1055 a; and thewidth of the third opening 1055 c is measured in the circumferentialdirection of the first opening 1055 a.

In a fifth step of the manufacturing method (i.e., (A5) and (B5) shownin FIGS. 12C and 12D), anisotropic etching is performed using the mask1055 on the wafer 1050 so as to form the hole 1011 and the recesses 1012and 1013 in the wafer 1050. Specifically, the prescribed portion of thewafer 1050 exposed from the mask 1055 is selectively removed by way ofanisotropic etching such as Deep-RIE. Herein, the width of the secondopening 1055 b and the width of the third opening 1055 c are bothsmaller than the diameter of the first opening 1055 a; and the width ofthe third opening 1055 c is smaller than the width of the second opening1055 b. Due to the aspect-dependent etching effect, the etching speedapplied to the second opening 1055 b and the third openings 1055 c ofthe wafer 1050 becomes slower than the etching speed applied to thefirst opening 1055 a. In addition, the etching speed applied to thethird openings 1055 c becomes slower than the etching speed applied tothe first opening 1055 a. Furthermore, the etching speed applied to thethird openings 1055 c becomes slower than the etching speed applied tothe second opening 1055 b. As a result, the recess 1012 is formed inconformity with the second opening 1055 b of the wafer 1050. Inaddition, the recess 1013 whose depth is smaller than the depth of therecess 1012 is formed in conformity with the third openings 1055 c ofthe wafer 1050.

Next, the mask 1055 is removed by use of a resist peeling solution suchas NMP (i.e., N-methyl-2-pyrolidone).

Next, the first film 1051 and the third film 1053, both of which aresilicon oxide films, are selectively removed by way of isotropic wetetching using an etching solution such as buffered hydrofluoric acid orby way of the combination of isotropic etching and anisotropic etching.At this time, the etching solution is supplied via the holes 1023 of thefourth film 1054 and the hole 1011 of the wafer 1050, thus dissolvingthe first film 1051 and the third film 1053. The openings 1014 and 1015are formed in the first film 1051 and the third film 1053 byappropriately designing the shapes and arrangements of the holes 1023and the hole 1011, thus forming the diaphragm 1020, the plate 1022, andthe support 1024 forming the sensing portion of the condenser microphone1001 (see FIG. 9).

Thereafter, the condenser microphone 1001 is completely produced by wayof dicing and packaging steps.

(a) First Variation

The manufacturing method of the condenser microphone 1001 can bemodified in a variety of ways. A first variation of the manufacturingmethod will be described with reference to FIGS. 13A to 13D.

Similar to the aforementioned manufacturing method, the first film 1051,the second film 1052, the third film 1053, and the fourth film 1054 aredeposited on the first surface 1050 a of the wafer 1050.

In a first step of the manufacturing method (i.e., (A1) and (B1) shownin FIGS. 13A and 13B), a mask 1255 is formed on the second surface 1050b of the wafer 1050. A plurality of second openings 1055 b are formed inthe periphery of the first opening 1055 a in the mask 1255. The distancebetween the adjacent second openings 1055 b is smaller than the distancebetween the first opening 1055 a and the second opening 1055 b.Specifically, the distance between the first opening 1055 a and thesecond opening 1055 b is greater than 20 μm, and the distance betweenthe adjacent second openings 1055 b is less than 20 μm.

Next, similar to the aforementioned manufacturing method, anisotropicetching is performed using the mask 1255 on the wafer 1050, thus formingthe hole 1011 and a plurality of recesses 1212 sequentially surroundingthe hole 1011 in the wafer 1050. A wall 1272 between the adjacentrecesses 1212 is thinner than a wall 1271 between the hole 1011 and therecess 1212.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 13C and 13D), the wall 1272 between the adjacent recesses 1212is removed so as to form the recess 1012 forming the second cavity 1042(see FIG. 9 and FIG. 10A) in the wafer 1050. Specifically, the secondsurface 1050 b of the monocrystal silicon wafer 1050 is subjected tothermal oxidation so as to transform the wall 1272 into silicon oxide.Next, wet etching is performed using an etching solution such asbuffered hydrofluoric acid so as to selectively remove the wall 1272.

Steps following the aforementioned steps are substantially identical tosteps of the aforementioned manufacturing method. In the first variationof the manufacturing method, a plurality of recesses 1212 are formed inthe wafer 1050 in such a way that the wall 1272 becomes thinner than thewall 1271. Herein, the recesses 1212 can be further modified inarrangement as long as the wall 1272 between the adjacent recesses 1212can be selectively removed relative to the wall 1271.

(b) Second Variation

The overall constitution of a condenser microphone according to a secondvariation of the second embodiment is substantially identical to theoverall constitution of the condenser microphone 1001 except for theshaping of a support 1024. The support 1024 forms a back cavityconstituted of the first cavity 1041 and a plurality of second cavities1042 communicating with the first cavity 1041. Herein, the secondcavities 1042 communicate with the first cavity 1041 via a plurality ofpassages 1043 having different resistances. This condenser microphone isproduced in such a way that the hole 1011, the plurality of recesses1012 each having a circular arc shape, and the recess 1013 extended fromthe hole 1011 to the recesses 1012 are formed in the second surface 1050b of the wafer 1050.

It is possible to delicately adjust output characteristics of thecondenser microphone by individually setting resistances of the passages1043 in response to required output characteristics. All of the secondcavities 1042 have the same volume, or they have different volumes. Itis possible to delicately adjust output characteristics of the condensermicrophone by individually setting the volumes of the second cavities.

(c) Other Variations

The second embodiment and its variations are each directed to thecondenser microphone serving as the pressure sensor, although the secondembodiment is applicable to other types of pressure sensors that detectpressure variations other than variations of sound pressure.

The second embodiment and its variations are each directed to thecondenser microphone in which the overall circumferences of thediaphragm 1020 and the plate 1022, each having a circular shape, arefixed to the support, although the second embodiment is not necessarilylimited in terms of the constitution of the sensing portion of thecondenser microphone constituted of the diaphragm and plate. Forexample, one end of the diaphragm and one end of the plate can be fixedto the support. In addition, both ends of the diaphragm can be fixed tothe support; alternatively, the diaphragm can be fixed to the support ina cantilever manner. The diaphragm and plate are not necessarily limitedin shape such as the circular shape. That is, the diaphragm and platecan be each shaped in a polygonal shape. Furthermore, the plate can bepositioned in proximity to the back cavity rather than the diaphragm.The diaphragm is not necessarily directly fixed to the support. That is,the diaphragm can be attached to the plate in a hung-down manner; or thediaphragm can be supported by the plate.

In the second embodiment and its variations, the second cavity formingportion is constituted of the recesses 1012 and 1013 formed on thesecond surface of the substrate 1010. The second cavity forming portioncan be formed using parts other than the support 1024. For example, thesecond cavity is arranged as a part of a package of the condensermicrophone, wherein the second cavity and the first cavity communicatewith each other via a passage formed in the substrate 1010.

In the second embodiment, the first cavity 1041 has a cylindrical shape.Of course, the first cavity 1041 is not necessarily formed in thecylindrical shape. The second cavity 1042 has a ring shape, although thesecond cavity 1042 can be redesigned to have a C-shape or a cylindricalshape. The passage 1043 is not necessarily limited to a linear shape andcan be bent appropriately.

In the second embodiment, the first cavity 1041 and the second cavities1042 communicate with each other via a plurality of passages 1043,although they can communicate with each other via a single passage.

In the second embodiment, the first cavity 1041 and the second cavities1042 communicate with each other via the passages 1043 having differentresistances, although they can communicate with each other via thepassages 1043 having the same resistance. Compared with the technologyin which the back cavity is constituted of a first cavity and a secondcavity, the second embodiment has an advantage in terms of the degree offreedom regarding the arrangement of second cavities.

3. Third Embodiment

FIGS. 14A and 14B show the constitution of a condenser microphone 2001in accordance with a third embodiment of the present invention. Thecondenser microphone 2001 is a silicon capacitor microphone that isproduced by way of semiconductor device manufacturing processes. Thecondenser microphone 2001 converts sound waves transmitted to a plate2030 into electric signals.

A sensing portion of the condenser microphone 2001 has a laminatedstructure in which first, second, third, and fourth films are laminatedtogether with a substrate 2010.

The substrate 2010 is a monocrystal silicon substrate. A through-hole2011 and a plurality of recesses 2012 are formed in the substrate 2010in its thickness direction. The through-hole 2011 is a cylindricalshape, which is opened at a first surface 2010 a and a second surface2010 b of the substrate 2010. Each of the recesses 2012 has achannel-like shape elongated externally of the through-hole 2011 in itsradial direction. The recesses 2012 are each opened on the secondsurface 2010 b of the substrate 2010. As a result, a gear-like opening2013, which is constituted of an opening 2013 a corresponding to thethrough-hole 2011 and a plurality of openings 2013 b corresponding tothe recesses 2012, is formed in the second surface 2010 b of thesubstrate 2010. The opening 2013 a (serving as a second opening) has acircular shape. Each of the openings 2013 b (serving as a third opening)has a rectangular shape elongated externally from the periphery of theopening 2013 a in its radial direction. An opening 2014 corresponding tothe through-hole 2011 is formed in the first surface 2010 a of thesubstrate 2010. The opening 2014 (serving as a first opening) has acircular shape substantially matching the circular shape of the opening2013 a.

The first film is an insulating thin film composed of silicon dioxide,wherein it has a through-hole 2015 having a cylindrical shape. The firstfilm supports the second film above the substrate 2010 in such a waythat a gap is formed between a diaphragm 2020 and the substrate 2010.

The second film is a conductive thin film composed of polysilicon dopedwith phosphorus (P) impurities. The prescribed portion of the secondfilm that is not fixed to the third film forms the diaphragm 2020. Thediaphragm 2020 is not fixed to either the first film or the third film,wherein it serves as a moving electrode vibrating due to sound waves.The diaphragm 2020 covers the through-hole 2015 of the first film. Thetwo-dimensional shape of the diaphragm 2020 is a circular shape.

Similar to the first film, the third film is an insulating thin filmcomposed of silicon dioxide, wherein it has a through-hole 2016 having acylindrical shape. The third film insulates the conductive second filmfrom the fourth film, and it supports the fourth film above the secondfilm.

Similar to the second film, the fourth film is a conductive thin filmcomposed of polysilicon doped with phosphorus (P) impurities. Theprescribed portion of the fourth film that is not fixed to the thirdfilm forms a plate 2030. The plate 2030 has a plurality of holes 2032.

A support 2040 is constituted of the substrate 2010, the first film, thethird film, and the prescribed portions of the second and fourth filmsthat are not fixed to the third film. The support 2040 forms a backcavity 2042 inwardly of the interior wall of the through-hole 2011 andthe interior wall of the through-hole 2015. The back cavity 2042releases pressure applied to the diaphragm 2020 in a direction oppositeto the propagation direction of sound waves. In the claim language, thesupport 2040 except for the substrate 2010 is defined as a support.

A detecting portion of the condenser microphone 2001 will be describedwith reference to the circuitry shown in FIG. 14A. Herein, the diaphragm2020 is connected to a bias voltage source. Specifically, leads 2104 and2106 connected to a terminal 2102 of the bias voltage source areconnected to the second film and the substrate 2010 respectively,whereby both of the diaphragm 2020 and the substrate 2010 are placed atsubstantially the same potential. The plate 2030 is connected to aninput terminal of the operational amplifier 2100. That is, a lead 2108connected to the input terminal of the operational amplifier 2100 havingrelatively high input impedance is connected to the fourth film.

Next, the operation of the condenser microphone 2001 will be describedin detail. When sound waves are transmitted through the holes 2032 ofthe plate 2030 to reach the diaphragm 2020, the diaphragm 2020 vibratesdue to sound waves. Due to the vibration of the diaphragm 2020, thedistance between the diaphragm 2020 and the plate 2030 is varied, sothat electrostatic capacitance between the diaphragm 2020 and the plate2030 is correspondingly varied.

Since the plate 2030 is connected to the operational amplifier 2100having relatively high input impedance, a very small amount of electriccharge existing in the plate 2030 moves toward the operational amplifier2100 irrespective of variations of electrostatic capacitance between thediaphragm 2020 and the plate 2030. That is, it is presumed that electriccharges existing in the plate 2030 and the diaphragm 2020 may besubstantially unchanged. This makes it possible to translateelectrostatic capacitance between the diaphragm 2020 and the plate 2030into potential variations of the plate 2030. Thus, the condensermicrophone 2001 is capable of producing electric signals in response tovery small variations of electrostatic capacitance between the diaphragm2020 and the plate 2030. In the condenser microphone 2001, variations ofsound pressure applied to the diaphragm 2020 are converted intovariations of electrostatic capacitance, which are then converted intopotential variations, based on which electric signals are produced inresponse to variations of sound pressure.

Next, a manufacturing method of the condenser microphone 2001 will bedescribed with reference to FIGS. 15A to 15F and FIGS. 16A to 16F.

In a first step of the manufacturing method (i.e., (A1) and (B1) shownin FIGS. 15A and 15B), a first film 2051 serving as a sacrifice film isdeposited on a wafer 2050 corresponding to the substrate 2010 (see FIGS.14A and 14B). Specifically, silicon dioxide is deposited on themonocrystal silicon wafer 2050 by way of plasma CVD, thus forming thefirst film 2051.

Next, a second film 2052 is deposited on the first film 2051.Specifically, phosphorus-doped polysilicon is deposited on the firstfilm 2051 by way of decompression CVD, thus forming the second film2052. Next, a photoresist film is applied to the entire surface of thesecond film 2052; then, a resist pattern is formed by way ofphotolithography for performing exposure and development using aprescribed resist mask. Then, the second film 2052 is selectivelyremoved by way of anisotropic etching such as RIE, thus shaping thesecond film 2052 having a circular shape.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 15C and 15D), a third film 2053 is deposited on the second film2052. That is, silicon dioxide is deposited on the second film 2052 byway of plasma CVD, thus forming the third film 2053.

In a third step of the manufacturing method (i.e., (A3) and (B3) shownin FIGS. 15E and 15F), a fourth film 2054 is deposited on the third film2053. Specifically, phosphorus-doped polysilicon is deposited on thethird film 2053 by way of decompression CVD, thus forming the fourthfilm 2054. Next, a photoresist film is applied to the entire surface ofthe fourth film 2054; then, a resist pattern is formed by way ofphotolithography for performing exposure and development using aprescribed resist mask. Then, the fourth film 2054 is selectivelyremoved by way of anisotropic etching such as RIE, thus shaping thefourth film 2054 having a circular shape and a plurality of holes 2022.

In a fourth step of the manufacturing method (i.e., (A4) and (B4) shownin FIGS. 16A and 16B), a mask 2055 having an opening 2055 a and aplurality of openings 2055 b is formed on a second surface 2050 bopposite to the first surface 2050 a of the wafer 2050 on which thefirst film 2051, the second film 2052, the third film 2053, and thefourth film 2054 are laminated together. That is, a photoresist mask isapplied entirely to the second surface 2050 b of the wafer 2050; then,the mask 2055 is formed by way of photolithography for performingexposure and development using a prescribed resist mask. The opening2055 a (serving as the first opening) has a circular shape in conformitywith the two-dimensional shape of the diaphragm 2020 (see FIG. 14A).Each of the openings 2055 b (serving as the second opening) has arectangular shape elongated from the periphery of the opening 2055 a inits radial direction. The openings 2055 b are formed in a radial mannerwith respect to the opening 2055 a. That is, the opening 2055 a and theopenings 2055 b collectively form a gear-like shape. The shorthand width(or slit width) of the opening 2055 b is much smaller than the diameterof the opening 2055 a. For example, the diameter of the opening 2055 aranges from 100 μm to 1000 μm, preferably, it is approximately set to600 μm; and the slit width of the opening 2055 b ranges from 1 μm to 50μm, preferably, it is approximately set to 40 μm.

In a fifth step of the manufacturing method (i.e., (A5) and (B5) shownin FIGS. 16C and 16D), anisotropic etching is performed using the mask2055 on the wafer 2050, thus forming the through-hole 2011 and therecesses 2012 in the wafer 2050. Specifically, the prescribed portion ofthe wafer 2050 exposed from the mask 2055 is selectively removed by wayof Deep-RIE. The anisotropic etching is continuously performed until thethrough-hole 2011 substantially matches the opening 2055 a of the mask2055 of the wafer 2050. Since the slit width of the opening 2055 b ismuch smaller than the diameter of the opening 2055 a, the etching speedapplied to the exposed portions of the openings 2055 b of the wafer 2050is slower than the etching speed applied to the exposed portion of theopening 2055 a of the wafer 2050 due to the aspect-dependent etchingeffect. Thus, the recesses 2012 are reliably formed in conformity withthe exposed portions of the openings 2055 b of the wafer 2050.

Next, the mask 2055 is removed by use of a resist peeling solution suchas NMP (N-methyl-2-pyrolidone).

In a sixth step of the manufacturing method (i.e., (A6) and (B6) shownin FIGS. 16E and 16F), wet etching is performed using an etchingsolution such as buffered hydrofluoric acid (or Buffered HF) so as toselectively remove the first film 2051 and the third film 2053, both ofwhich are silicon oxide films. The etching solution is introduced viathe through-hole 2011 and the recesses 2012 of the wafer 2050 as well asthe holes 2032 of the fourth film 2054, thus dissolving the first film2051 and the third film 2053. By appropriately designing the shapes andarrangements of the through-hole 2011 and the holes 2032, it is possibleto form the through-holes 2015 and 2016 in the first film 2051 and thethird film 2053, whereby it is possible to form the sensing portionconstituted of the diaphragm 2020, the plate 2030, and the support 2040(see FIG. 14A). The diaphragm 2020 has a circular shape incorrespondence with the through-hole 2011, which is shaped in conformitywith the opening 2014 of the first surface 2050 a of the wafer 2050. Theaforementioned process will be referred to as a wet etching process.

Thereafter, dicing and packaging steps are performed, thus, it ispossible to completely produce the condenser microphone 2001.

In the third embodiment, the opening 2013 of the first surface 2050 a ofthe substrate 2010 has a gear-like shape constituted of the through-hole2011 and the recesses 2012. Even when bubbles occur so as to entirelycover the opening 2013 (i.e., an inlet opening for introducing anetching solution) in the wet etching process, surface tensions areunevenly distributed to bubbles due to the rectangular openings 2013 b,which are elongated from the periphery of the opening 2013 a having acircular shape in a radial direction; hence, bubbles may be easilyburst. This simplifies the manufacturing process of the condensermicrophone 2001.

In the third embodiment, the openings 2013 b are formed on the secondsurface 2010 b of the substrate 2010 by means of the recesses 2012,which are not opened in the first surface 2010 a of the substrate 2010.This makes it possible to form the opening 2014 on the first surface2010 a of the substrate 2010 in correspondence with the two-dimensionalshape of the diaphragm 2020 irrespective of the shape of the opening2013 of the second surface 2010 b of the substrate 2010. For thisreason, it is possible to prevent output characteristics of thecondenser microphone 2001 from being degraded.

The third embodiment can be further modified in a variety of ways;hence, variations will be described below.

(a) First Variation

FIGS. 17A and 17B show the constitution of a condenser microphone 2002in accordance with a first variation of the third embodiment. Thecondenser microphone 2002 has a substrate 2210 having a firstthrough-hole 2211. All the constituent elements of the condensermicrophone 2002 are substantially identical to those of the condensermicrophone 2001 except for the substrate 2210 forming the sensingportion.

As shown in FIGS. 17A and 17B, the substrate 2210 is a monocrystalsilicon substrate, in which the first through-hole 2211 and secondthrough-holes 2212 are formed in a thickness direction. An opening 2213having a gear-like shape constituted of an opening 2213 a (correspondingto the first through-hole 2211) and a plurality of openings 2213 b(corresponding to the second through-holes 2212) is formed in a secondsurface 2210 b of the substrate 2210. The opening 2213 a (serving as asecond opening) has a circular shape. Each of the openings 2213 b(serving as fourth openings) has a rectangular shape elongated from theperiphery of the opening 2213 a in its radial direction. On the otherhand, a first opening (corresponding to the first through-hole 2211) anda plurality of third openings (corresponding to the second through-holes2212) are formed in a first surface 2210 a of the substrate 2210.Herein, the first opening is shaped substantially in conformity with theopening 2213 a, and the third openings are shaped substantially inconformity with the openings 2213 b.

Next, a manufacturing method of the condenser microphone 2002 will bedescribed with reference to FIGS. 18A to 18D. Similar to themanufacturing method of the condenser microphone 2001, the first film2051, the second film 2052, the third film 2053, and the fourth film2054 are deposited on the first surface 2050 a of the wafer 2050 formingthe substrate 2210.

In a first step of the manufacturing method (i.e., (A1) and (B1) shownin FIGS. 18A and 18B), a mask 2255 having an opening 2255 a (serving asthe first opening) and a plurality of openings 2255 b (serving as thesecond openings) is formed on the second surface 2050 b opposite to thefirst surface 2050 a of the wafer 2050. The mask 2255 is substantiallyidentical to the mask 2055 so that the openings 2255 a and 2255 bsubstantially match the openings 2055 a and 2055 b, although the widthsof the openings 2255 b can be adequately increased so that the etchingspeed applied to the exposed portion of the opening 2255 a becomessubstantially identical to the etching speed applied to the exposedportions of the openings 2255 b. For example, the diameter of theopening 2255 a ranges from 100 μm to 1000 μm, preferably, it isapproximately set to 600 μm; and the width of the opening 2255 b rangesfrom 40 μm to 200 μm, preferably, it is approximately set to 100 μm.

In a second step of the manufacturing method (i.e., (A2) and (B2) shownin FIGS. 18C and 18D), anisotropic etching is performed using the mask2255 on the wafer 2050 so as to form the first through-hole 2211 and thesecond through-holes 2212 in the wafer 2050. Specifically, theprescribed portion of the wafer 2050 exposed from the mask 2255 isselectively removed by way of Deep-RIE. Since substantially the sameetching speed is applied to both of the exposed portion of the opening2255 a and the exposed portions of the openings 2255 b, the firstthrough-hole 2211 is formed in conformity with the opening 2255 a, andthe second through-holes 2212 are formed in conformity with the openings2255 b in the wafer 2050.

Steps following the aforementioned steps are substantially identical tothose of the manufacturing method of the condenser microphone 2001.

In the third embodiment, the opening 2213 formed on the second surface2210 b of the substrate 2210 has a gear-like shape constituted of thefirst through-hole 2211 and the second through-holes 2212. As a result,even when bubbles occur to entirely cover the opening 2213, which is aninlet opening for introducing an etching solution, in the wet etchingprocess, surface tensions are unevenly distributed to bubbles by meansof the rectangular openings 2213 b elongated from the periphery of thecircular opening 2213 a in its radial direction; hence, bubbles may beeasily burst. This simplifies the manufacturing process of the condensermicrophone 2002.

In the first variation of the third embodiment, a circular opening isformed in conformity with the two-dimensional shape of the diaphragm2020 on the first surface 2210 a of the substrate 2210 by means of thefirst through-hole 2211. By appropriately designing the secondthrough-holes 2212, it is possible to prevent output characteristics ofthe condenser microphone 2002 from being degraded.

(b) Other Variations

The third embodiment and its first variation are each directed to thecondenser microphone as an example of the pressure sensor. Of course,the third embodiment can be applied to other types of pressure sensorsthat detect pressure variations other than variations of sound pressure.

In the third embodiment and its first variation, a gear-like opening isformed on the second surface of the substrate positioned opposite to thediaphragm, whereas the opening formed on the second surface of thesubstrate is not necessarily formed in a gear-like shape. For example,it is possible to produce a condenser microphone 2003 as shown in FIGS.19A and 19B, in which an opening 2313 constituted of an opening 2313 a(that is shaped in conformity with the two-dimensional shape of thediaphragm) and a plurality of openings 2313 b (having triangular shapesthat project externally of the periphery of the opening 2313 a) isformed in the second surface o a substrate 2310.

Incidentally, the third embodiment and its variations are all directedto the condenser microphone having the circular diaphragm 2020, althoughthe two-dimensional shape of the diaphragm 2020 is not necessarilylimited to the circular shape. For example, the opening 2013 a can beformed in a prescribed shape other than the circular shape in conformitywith the two-dimensional shape of the diaphragm 2020 in the condensermicrophone 2001. Similarly, the opening 2213 a can be formed in aprescribed shape other than the circular shape in conformity with thetwo-dimensional shape of the diaphragm 2020. Lastly, the presentinvention is not necessarily limited to the aforementioned embodimentsand variations; hence, it can be further modified within the scope ofthe invention defined by the appended claims.

1. A pressure sensor comprising: a plate having a fixed electrode: adiaphragm having a moving electrode positioned opposite to the fixedelectrode, wherein the diaphragm is subjected to displacement due topressure variations applied thereto; and a support having an opening, onwhich end portions of the plate are fixed, and an interior wall, inwhich a step portion is formed and which forms a cavity whosecross-sectional area is larger than a cross-sectional area of theopening in any plane of the cavity parallel to the plate.
 2. Thepressure sensor according to claim 1, wherein the cross-sectional areaof the cavity in a plane direction parallel to the plate is enlarged ina direction opposite to the plate by way of the step portion, which theinterior wall forms in the thickness direction of the diaphragm.
 3. Apressure sensor comprising: a plate having a fixed electrode: adiaphragm having a moving electrode positioned opposite to the fixedelectrode, wherein the diaphragm is subjected to displacement due topressure variations applied thereto; a support having an interior wall,which end portions of the plate are fixed to, wherein a first cavity isformed inwardly of the interior wall of the support and the diaphragm;and a sub-cavity forming portion for forming a second cavitycommunicating with the first cavity via a passage having an openingcommunicating the first cavity.
 4. The pressure sensor according toclaim 3, wherein the sub-cavity forming portion is arranged in thesupport, and wherein the passage and the second cavity are formedinwardly of a recess of the support.
 5. The pressure sensor according toclaim 3, wherein the sub-cavity forming portion forms a plurality ofsecond cavities and a plurality of passages having differentresistances, via which the first cavity communicates with the secondcavities.
 6. The pressure sensor according to claim 4, wherein thesub-cavity forming portion forms a plurality of second cavities and aplurality of passages having different resistances, via which the firstcavity communicates with the second cavities.
 7. The pressure sensoraccording to claim 5, wherein the plurality of second cavities havedifferent volumes.
 8. The pressure sensor according to claim 6, whereinthe plurality of second cavities have different volumes.
 9. A pressuresensor comprising: a substrate having a first surface and a secondsurface, which are positioned opposite to each other; a plate having afixed electrode, which is constituted of a thin film formed on the firstsurface of the substrate; a diaphragm having a moving electrodepositioned opposite to the fixed electrode, wherein the diaphragm isconstituted of a thin film formed on the first surface of the substrateand is subjected to displacement due to pressure variations appliedthereto; a support constituted of a thin film, which is composed of amaterial that can be selectively removed from the substrate by way ofwet etching and which is formed on the first surface of the substrate,wherein the support supports the plate such that a gap is formed betweenthe fixed electrode and the moving electrode; a through-hole that isformed to run through the substrate in its thickness direction so as toexpose the diaphragm, wherein the through-hole has a first opening,which is formed on the first surface of the substrate in conformity witha two-dimensional shape of the diaphragm, and a second opening whoseshape is substantially identical to a shape of the first opening andwhich is formed on the second surface of the substrate; and a recess,which is formed on the second surface of the substrate and which forms athird opening communicating with the second opening in its periphery.10. A pressure sensor comprising: a substrate having a first surface anda second surface, which are positioned opposite to each other; a platehaving a fixed electrode, which is constituted of a thin film formed onthe first surface of the substrate; a diaphragm having a movingelectrode positioned opposite to the fixed electrode, wherein thediaphragm is constituted of a thin film formed on the first surface ofthe substrate and is subjected to displacement due to pressurevariations applied thereto; a support constituted of a thin film, whichis composed of a material that can be selectively removed from thesubstrate by way of wet etching and which is formed on the first surfaceof the substrate, wherein the support supports the plate such that a gapis formed between the fixed electrode and the moving electrode; a firstthrough-hole that is formed to run through the substrate in itsthickness direction so as to expose the diaphragm, wherein the firstthrough-hole has a first opening, which is formed on the first surfaceof the substrate in conformity with a two-dimensional shape of thediaphragm, and a second opening whose shape is substantially identicalto a shape of the first opening and which is formed on the secondsurface of the substrate; and a second through-hole that is formed torun through the substrate in its thickness direction, wherein the secondthrough-hole forms a third opening communicating with the first openingin its periphery on the first surface of the substrate and a fourthopening whose shape is substantially identical to a shape of the thirdopening on the second surface of the substrate.